Overview of C51 Language

1) Advantages: C51 is a high-level programming language designed for the 51 microcontroller and is a subset of standard C language. It features strong readability, easy debugging and maintenance, and a small programming workload. It allows direct access to physical addresses, enabling direct operations on hardware and achieving some functionalities of assembly language, thus combining features of both high-level and low-level languages, making it widely applicable. Currently, C51 has become the mainstream programming method for developing programs for the 51 microcontroller.

Structured language, compact code – efficiency comparable to assembly language (but still not as good).

Close to real language, strong program readability – easy to debug and maintain.

Rich library functions, small programming workload – short product development cycle.

Strong machine-level control capability – suitable for embedded system development.

Independent of assembly instructions, easy to master – quick to get started on the microcontroller.

2) Disadvantages: Execution efficiency is not as good as assembly language.

Source programs written in C51 cannot be directly recognized by the computer; they must be converted into executable language (or target code) before execution.

The method of converting the high-level language source program into target code entirely before execution is called compiled execution.

C51 uses compiled execution, and the generated target code can be executed independently of the C51 programming environment, resulting in fast execution speed and high code efficiency.

The method of converting and executing the source program simultaneously is called interpreted execution.

Similarities: Syntax rules, program structure, programming methods.

Differences: Data structures, interrupt handling, port expansion.

“Create a new project file” → “Select microcontroller” → “Edit source program” → “Add source program” → “Modify project configuration” → “Compile program” → “Generate executable file”.

· The basic unit of a C51 program is a function; a C51 source program must contain at least one main function, which can also include several other functions.

· The main function is the entry point of the program;

· When all statements in the main function have been executed, the program ends.

Example: LED blinking control function.

1. Variables in C51

a) In C language programming, a quantity that can change is called a variable.

b) The basic attributes of a variable are its name and value.

c) When defining a variable in a program, the C51 compiler allocates the corresponding storage unit for that variable, thus binding the variable name with the address of the storage unit, and the variable value corresponds to the content of the storage unit (if different content is placed in the storage unit, the variable will have different values).

d) The process of using a variable is to find the corresponding memory address through the variable name and then perform data reading operations on that storage unit.

Factors to Consider When Using Variables:

a) If a variable needs to correspond to larger values, it requires linking multiple consecutive storage units to increase the bit count, which involves the issue of variable data types.

b) The 51 microcontroller has both internal RAM and external RAM, which involves the issue of variable storage types.

c) Permanently binding variables to storage units reduces the utilization of storage units; dynamically allocating storage units temporarily reduces the convenience of using variables, which involves the issue of variable storage categories.

Complete format for defining a variable (4 main elements):

[Storage category] Data type [Storage type] Variable name;

Variable definition must also consider the variable’s scope.

To improve variable storage efficiency, a more scientific approach is:

① For variables that only have current usage value, they can be automatically released after use to allow the compiler to reallocate variable storage space; — auto

② For variables with long-term usage value, they can be kept under static protection and not released during program execution; — static

③ For variables that need to pass data between multiple programs or functions, they can be defined in one place, with their external attributes declared in other programs or functions to achieve data sharing; — extern

④ For variables that need to frequently change their values, their values can be stored in the CPU’s registers to avoid repeated memory access, thus achieving higher execution efficiency. — register

There are 4 types of variable storage:

1. Automatic type (auto).

Variables with the auto attribute are called automatic variables. The scope of automatic variables is within the function body or statement block where they are defined. When the function call ends or the statement block execution is complete, the storage unit occupied by automatic variables is released. Since the values in the storage unit are random, the initial value of automatic variables before assignment is also random. Automatic is the default option for “storage category”; if the “storage category” item is omitted when defining the variable, the variable is assumed to be automatic.

2. Static type (static).

Variables with the static attribute are called static variables. The scope of static variables is within the function body, program file, or statement block where they are defined. Static variables have three characteristics: variable hiding, persistent storage, and default initial value of 0.

If you want a variable to retain its value after leaving the scope, or to prevent it from being used by other variables with the same name outside the scope, or to ensure that a variable defined without an initial value defaults to 0, you can declare it with static during variable definition.

3. External type (extern).

Variables with the extern attribute are called external variables. If the definition and use of a variable are not in the same scope, it can be extended to the declaration location using extern, thereby bringing the variable value into the new scope. The extensibility of extern is the opposite of the hiding nature of static. After a variable is declared extern, it can be allocated a fixed storage unit and remain valid throughout the program’s execution period.

4. Register type (register).

Variables with the register attribute are called register variables. If a variable needs to frequently exchange data with memory during use, its storage unit can be designated as a register using register. However, with continuous optimization of compiler technology, compilers can now automatically place frequently exchanged variables in registers, reducing the necessity of register declarations.

Note:

The keywords auto, static, and register must be used simultaneously during variable definition and cannot be used alone.

Therefore, the following usage is incorrect:

int a; (defining integer variable a first)

static a; (attempting to declare a as a static variable again)

This will be considered a “redefinition” by the compiler. The correct usage should be: static int a;

However, extern can be declared alone; it can extend the scope of an already defined variable, for example:

int b; (defined variable b in another program file or function)

extern b; (extends the scope of variable b here, allowing the omission of its int type)

The issue of variable definition and variable declaration. In practice, these two are often used interchangeably, which is not strict because there is a distinction between them.

Variable definition involves both the specification of variable characteristics and the allocation of storage units.

Variable declaration only indicates the characteristics of the variable without involving the allocation of storage units.

For example, the previously mentioned “int a;” is called variable definition, while “extern b;” is called variable declaration.

According to C51 rules, the position of the variable definition statement determines the variable’s scope, where those placed at the beginning of the program (i.e., before all functions) are called global variables, while those placed inside functions are called local variables.

The scope of global variables is the entire source program, and their values remain valid throughout program execution, while the values of local variables are only valid during function calls and will become invalid after the call ends.

To make reasonable use of storage resources, it is necessary to flexibly adopt global or local variables based on the situation, generally preferring local variables.

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